The term "particulates" for purposes of the present invention include individual or equiaxed particles, platelets, flakes, whiskers and short or chopped fibers. It is common to use particulates as additives, reinforcements and functional elements in plastics, rubbers, metals, metal alloys, ceramics and other materials to form composites having improved properties.
The characteristics and surface properties of composite particulates can be further enhanced to improve their resistance to corrosion, moisture and/or heat etc. by coating the composite particulates with a metallic composition. The coating can also be used to provide enhanced surface characteristics and surface texture. This represents yet another generation of composite particulates which have many important applications in different areas of technology.
There are numerous conventional coating processes available to coat metal upon particulates including electroplating, which is also commonly referred to as electrodeposition, chemical vapor deposition (CVD), physical vapor deposition (PVD) and autocatalytic (electroless) plating. Electroplating is preferred as being more versatile in the selection of metal to be coated, high coating efficiency relative to the other processes, control over coating thickness and cost. Nevertheless, at present, the electroplating process has limited commercial application for reasons related primarily to its inability to uniformly coat particulates of wide varying sizes and the coating speed to form a coating of given thickness is low. In fact, at present, the electroplating process typically requires a total electroplating time of about 100 hours or more to coat an average thickness of 1.0 .mu.m. For many commercial applications this is unacceptable.
Electroplating metal coating onto the surface of particulates is taught in U.S. Pat. No. 4908106. This patent is limited to particulates having a small size i.e. "fine" particulates in a size range varying from 0.1 to 10 .mu.m and at low current density of between 2A/dm.sup.2 to 5 A/dm.sup.2 of the cathode plate. For consistency all reference hereafter to current density for purposes of this patent application relates to a measurement of current in amperes per surface area (dm.sup.2) of cathode plate. Heretofore electroplating was carried out at current densities below about 5A/dm.sup.2 and required a very long electroplating time to coat a given amount of metal on the particulates.
Electroplating of metal coatings on particulates is also taught in Japanese Patent No's: JP-A-59-41489 and JP-A59-89788 respectively. Both of these Japanese patents teach an electroplating process which requires the particles to be suspended in an electrolyte solution which is continuously agitated while performing the electroplating operation. The electroplating operation is conducted at low current density in the range of 0.4 A/dm.sup.2 .about.1.7 A/dm.sup.2 which was calculated based upon its teaching of current per gram particulates, particulate loading and diameter of the cathode plate. Although the current amperage through the electrolyte can be intentionally increased, if this were done in the arrangement taught in these Japanese patents only part of the current available can reduce the metallic ions on the particles whereas the rest of the current would be wasted on the generation of hydrogen and the heating of the electrolyte solution. Operation at low current density results in low coating speed i.e. it requires a longer total electroplating time to deposit a given volume of metal or a given average coating thickness.
In many applications, such as metal-matrix composites and thermal spray powder, sufficient coating thickness (&gt; at least 0.5 .mu.m) is needed and typically well over 1.0 .mu.m. Because of the large specific surface area of the particulates even thin coatings require a deposit of a relatively large amount of metal. If a low plating current density below 5 A/dm.sup.2 is used as taught in the prior art the coating rate or speed will be comparatively low requiring long electroplating times involving many days of electroplating which is not cost effective for large volume applications.